This invention relates to a rolling bearing for use in a hand-piece of a dental instrument for boring and cutting teeth.
Some conventional hand-pieces for dental instruments can rotate dental cutting tools mounted on their shafts at a ultra-high speed of 300000 rpm or higher.
FIG. 1 shows a conventional such hand-piece. It comprises a shaft 4 to which a dental cutting tool 3 is detachably mounted, a turbine 5 secured to the shaft 4, and a passage 6 for introducing compressed air for driving the turbine 5. The shaft 4 is rotatably supported by a rolling bearing 2. A passage 7 introduces pressurized water.
Such a hand-piece is required to be easy to handle, lightweight and quiet in operation. Thus, its bearing assembly also has to be lightweight and quiet. Thus, its retainer for rolling elements is formed e.g. from a polyamide resin, a material known for its high lubricity and flexibility.
Since such a hand-piece is used in the mouth of a patient, the amount of a lubricant used for lubricating its bearing assembly has to be as small as possible. Thus, conventional bearing assemblies of this type were lubricated by dripping a small amount of lubricating oil, or with oil mist mixed and diluted with compressed air.
Such a hand-piece is exposed to a hot (135.degree. C. or higher) vapor for sterilization (or autoclaving). Today, such sterilization is carried out at an even higher temperature or using a solvent to prevent infection with the HIV virus in particular. Thus, such a hand-piece is required to have a high chemical resistance and withstand temperatures ranging from 135.degree. C. to about 200.degree. C.
However, conventional dental hand-pieces having a synthetic resin retainer are unsatisfactory in heat resistance, so that they cannot withstand sterilizing temperatures higher than 135.degree. C.
In Examined Japanese Patent Publication 5-43884, the applicant of this invention proposed a rolling bearing for use in aeronautical and space equipment. Its retainer is made of a polyamideimide resin and impregnated with a completely fluorinated oil. But since the porous polyamideimide resin used in this publication is a melt type resin, there is the possibility that the retainer may melt and the bearing become unusable if the supply of lubricating oil stops. Also, due to its high moisture-absorbing speed, extreme care must be taken to control the quality of the material powder and end products.
In short, there was no rolling bearing assembly for use in a dental hand-piece which can withstand temperatures exceeding 135.degree. C., which can be used at ultra-high speed of 300000 rpm or higher, while requiring only a very small amount of lubricant, and which can be formed from a material which can be handled easily.
An object of this invention is to provide a rolling bearing assembly for use in a hand-piece which can withstand sterilization at temperatures of 135-200.degree. C., which can be rotated at high speeds of 300000 rpm and higher with a minimum consumption of lubricant, and whose retainer never melts even if the supply of lubricant stops completely.